Polypropylene has been widely used in various molded articles, films and sheets because of its excellent properties in moldability, toughness, water resistance, gasoline resistance, chemical resistance, and the like.
However, polypropylene has difficulties or defects which need to be overcome in heat resistance, rigidity, impact resistance, scratch resistance, coating properties, adhesion properties, and printability. These problems have been a bar to broadening of application of polypropylene.
In order to improve coating properties, adhesion properties, and printability, it has been proposed to undergo graft-modification of a part or the whole of polypropylene with an unsaturated carboxylic acid or an anhydride thereof, e.g., maleic anhydride, as disclosed, e.g., in JP-B-58-47418 and JP-A-58-49736 (the terms "JP-A" and "JP-B" as used herein mean an "unexamined published Japanese patent application" and an "examined published Japanese patent application", respectively.). However, such modified polypropylene is still unsatisfactory in impact resistance, heat resistance, rigidity, and other physical properties.
For the purpose of improving compatibility between polypropylene and adhesives or coatings, a medium layer called a primer typically comprising, for example, chlorinated polypropylene and toluene is usually provided therebetween. However, because a primer itself is expensive and, also, an additional step should be involved, provision of a primer results in an increase in cost of final products.
Surface pretreatments before coating, printing or adhering which have been proposed to date include sandblasting, treatment with a chromic acid mixed solution, flame treatment, corona discharge treatment, plasma treatment, treatment for introducing a surface functional group, and surface light grafting, but any of these conventional treatments has not achieved satisfactory results as discussed below.
Sandblasting which consists of striking a granular abrasive against a molded article at a high speed to roughen the surface is accompanied by contamination of the working environment or products with the abrasive particles. It is therefore necessary to wash the surface of the sandblasted molded article with water before coating. Moreover, the surface of the article becomes opaque by sandblasting, and the abrasive which has eaten into the surface cannot be removed.
The treatment with a chromic acid mixed solution consists of immersing a molded article in a chromic acid mixed solution comprising 75 parts by weight of potassium bichromate, 120 parts of water, and 1,500 parts of concentrated sulfuric acid heated at about 100.degree. C. for about 5 minutes. This treatment requires a heavy burden in making the waste liquid harmless.
Flame treatment which consists of treating the surface of a molded article with an oxidizing flame of a gas having incorporated therein excess air (1000.degree. to 2500.degree. C.) sometimes causes deformation or fusion of the article by heat.
Corona discharge treatment which consists of passing a film between an electrode and a metallic roll and applying a high voltage thereto cannot be applied to articles other than films.
Plasma treatment which consists of applying a low-temperature plasma onto the plastic surface to induce a chemical change by an ionized gas and ultraviolet rays uses a plasma of oxygen or air. This treatment is disadvantageous in that the cost required for equipment is high.
The treatment of introducing a surface polyfunctional group is carried out by, for example, irradiating ultraviolet rays on the surface of an article in chlorine gas followed by treating with an alkali. Use of chlorine gas constitutes much danger.
Surface light grafting is carried out by, for example, incorporating benzophenone into a polypropylene film and light-graft polymerizing acrylamide in an atmosphere blocked from oxygen. From the economical consideration, this method is disadvantageous in that the steps involved are complicated.
These conventional surface treatments have various problems as discussed above, and there has thus been a strong demand for development of a new technique of surface treatment.
On the other hand, while polyamide resins, polyphenylene ether resins, and saturated polyester resins have been widely used as engineering resins characterized by their heat resistance, rigidity, strength, and oil resistance in the field of automobile parts and electric and electronic parts, there has been a demand for further improvements in moldability, impact resistance, water resistance, and chemical resistance. In addition, these resins have essential disadvantages of higher specific gravity and higher price as compared with polyolefin resins.
Under the above-mentioned situation, a resin composition comprising a polypropylene resin selected from modified polypropylene and a modified polypropylene/polypropylene composition and at least one engineering resin selected from a polyamide resin, a polyphenylene ether resin, and a saturated polyester resin and thereby exhibiting excellent properties characteristic of both resin components is expected to have a broadened application.
Notwithstanding the expectation, a polypropylene resin and the above-mentioned engineering resins have been regarded extremely poor in compatibility and dispersibility with each other. In fact, a mere blend of these resin components has the following defects.
1) The molten polymer blend undergoes a considerable Barus effect, making it nearly impossible to take up an extruded strand in a stable manner, thus remarkably reducing molding workability.
2) The blend is injection molded with extreme non-uniformity only to produce an injection molded product having a poor appearance due to flow marks which withstands no practical use in automobile parts and electric and electronic parts.
3) Molded particles obtained from a blend of a polypropylene resin and a polyamide resin frequently show lower physical properties, and particularly impact resistance and tensile elongation than those expected from synergism of the two resin components.
With the method described in JP-A-61-64741 being followed, while polypropylene and polyamide are essentially incompatible with each other, a polypropylene resin selected from modified polypropylene and a modified polypropylene/polypropylene composition and a polyamide resin can be dispersed together with good compatibility by incorporating thereto an epoxy-containing copolymer to produce a thermoplastic resin composition having well-balanced physical properties, such as moldability, rigidity, heat resistance, impact resistance, scratch resistance, oil resistance, chemical resistance, and water resistance, as well as a uniform and smooth appearance.